In this blog post, we’ll examine the problem of sandy beaches disappearing due to coastal erosion, the principles and potential applications of submerged breakwaters, and their impact on the coastal environment.
Everyone probably has memories from childhood of building sandcastles on the beach. You likely also have fond memories of spending hours playing in the sand with friends during trips to the beach as a child. However, if we’re not careful, we may no longer be able to build sandcastles in the future. This is because the area of sandy beaches is gradually shrinking year by year. Experts cite artificial structures along the coast, such as breakwaters and retaining walls, as causes of coastal erosion. So, how does coastal erosion affect us?
Haeundae Beach, one of South Korea’s most renowned beaches, has long faced issues with sand loss. In 1947, Haeundae Beach’s sandy shore was approximately 58.6 meters wide and covered an area of 88,658 square meters; however, by 2007, it had shrunk to 42.5 meters in width and 62,129 square meters in area, according to surveys. The blocking of sand inflow due to the construction of the beach road and the reclamation of Suyeong Bay was also cited as a cause of coastal erosion. In the past, efforts were made to restore the sandy beach by depositing significant amounts of sand into the sea each year to address this issue; however, as the deposited sand repeatedly eroded away, concerns were raised that a fundamental solution was needed.
Busan’s Songdo Beach, Korea’s first public beach which opened in 1913, has also suffered from coastal erosion and sand loss. In response, Busan’s Seo-gu District has been implementing the Songdo Coastal Restoration Project since 2000, investing a total of 43 billion won, including national and municipal funds. As part of this effort, improvements were made, such as the installation of an underwater breakwater, known as a “submerged breakwater,” in the waters off the beach. A submerged breakwater is a coastal structure used to reduce wave energy and minimize the loss of sand moving toward the coast.
A permeable underwater breakwater works by reducing wave energy without completely blocking the flow of water, allowing a certain amount to pass through. The advantages of this structure are as follows.
1. Since the intake opening of the channel is located near the surface on the open sea side, it reduces the risk of impaired seawater circulation or burial due to sediment deposition that could result from making the channel excessively deep.
2. Since the harbor side of the channel is located near the seabed, it enhances the wave-blocking effect.
3. Asymmetry in the flow inside and outside the channel promotes seawater exchange.
In other words, it is an underwater structure that maintains a certain degree of free water flow while reducing sand loss; therefore, it can help reduce the amount of sand washed out to sea from the beach and mitigate the impact of typhoons or high waves. Furthermore, there have been expectations that it could also reduce “rip currents”—fast-moving currents that carry water rapidly away from the shore in a short period of time.
A study commissioned by Haeundae District from a specialized agency at a cost of approximately 1 billion won to restore the sandy beach found that seasonal winds have a significant impact on sand movement at Haeundae Beach. The study found that easterly winds blow at Haeundae Beach in the winter and southerly winds in the summer; it was confirmed that the winter easterly winds sweep sand toward Dongbaek Island, while the summer southerly winds sweep sand toward Mipo Pier out to sea. Consequently, the research team concluded that installing underwater breakwaters near Dongbaek Island and Mipo Pier could reduce sand loss. Specifically, this involves installing “tetrapods”—tri-legged concrete structures—on the seabed to weaken the force of waves washing ashore at the beach.
However, there is significant opposition arguing that even if such underwater breakwaters can temporarily prevent sand loss, they may cause more harm than good in the long run. The argument is that if submerged breakwaters are installed at both ends of the beach, as in Haeundae, sand will accumulate around the breakwaters while being eroded from the center of the beach, which could actually alter the coastline. In fact, during the planning phase for the submerged breakwater in Haeundae, model experiments showed that simply adding sand from outside sources could cause erosion in the central part of the beach and sedimentation in Mipo Port, sparking controversy over the breakwater’s effectiveness and environmental impact. Furthermore, concerns were raised that changes in ocean currents could contribute to coastal erosion or other natural disasters. In other words, the potential for secondary erosion caused by these structures cannot be ignored. Consequently, some argue that it is more important to find methods that enhance natural resilience—such as installing bamboo traps to prevent sand from being washed away, as was done at Gijipo Beach in Taean, South Chungcheong Province.
Accordingly, the Coastal Engineering Research Laboratory (CoastEng) is conducting research—using a combination of numerical simulations and hydraulic model experiments—on a type of breakwater that dissipates wave energy using a permeable design, reduces both reflected and transmitted waves, and simultaneously allows for seawater circulation, thereby improving water quality within the harbor. Furthermore, rather than relying on traditional deterministic methods, the research applies a reliability-based design approach for coastal structures—grounded in probability theory—to account for statistical uncertainties in loads and strength during structural design, thereby achieving optimal designs while avoiding overdesign.
Therefore, rather than rushing to find short-term solutions, it appears necessary to develop restoration measures that minimize adverse environmental impacts through continuous coastal monitoring. This is because coastal erosion is not merely a matter of sand disappearing, but a complex phenomenon in which waves, currents, sediment transport, and coastal structures all interact with one another. Therefore, rather than attempting to solve the problem simply by installing specific structures, it is necessary to consider methods that maximize the coast’s natural resilience based on long-term observation and analysis. In this way, we can restore the sandy beaches of the past and create a space filled with memories that future generations can share with our own.
